Method for removing soot particles from an exhaust gas, associated collecting element and system

ABSTRACT

A method for removing soot particles from an exhaust gas of an internal combustion engine, especially of a diesel engine, includes feeding the exhaust gas through a collecting element through which the exhaust gas can pass freely but which is provided with a plurality of deflections and/or zones of swirl and calming or stabilization. At least a proportion of the particles are held or swirled around in the collecting element until there is a sufficient probability of reaction with nitrogen dioxide and a majority of the collected particles have been removed. A collecting element has flow channels through which the exhaust gas can pass freely. However, the flow channels are configured in such a way as to form deflections or zones of swirl and calming or stabilization. A system having the collecting element is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of application Ser. No.10/281,003 filed Oct. 25, 2002, which is a continuation of copendingInternational Application No. PCT/EP01/04486, filed Apr. 20, 2001, whichdesignated the United States and was not published in English. Thisapplication also claims the priority, under 35 U.S.C. §119, of GermanPatent Application DE 100 20 170.9, filed Apr. 25, 2000; the priorapplications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for removing soot particles from anexhaust gas, in particular of an internal combustion engine. Theinvention also relates to a collecting element suitable for thatpurpose, in particular one which can be regenerated and can be installedin a pipe, e.g. in an exhaust line of a motor vehicle. The inventionadditionally relates to a system having a collecting element.

In addition to gaseous constituents, the exhaust gas of an internalcombustion engine also contains particles. Those particles enter theenvironment directly with the exhaust gas, which is undesirable, or theyare often deposited somewhere in the exhaust system, with the resultthat they are then emitted in the form of a cloud of particles in thecase of load changes, for example.

It is conventional to use filters to collect the particles. However, theuse of closed filter systems entails two significant disadvantages: onone hand, those filters can become clogged and, on the other hand, theycause an unwanted pressure drop. If no filters are used, those particleswhich do not pass directly into the environment may be deposited on thecoating of an installed catalytic converter which is provided forpurifying the exhaust gas of other polluting components, where they leadto poisoning or at least to a reduction in the catalytically activesurface area. As part of ever-stricter laws to protect the environment,emissions of pollutants and particles are to be reduced further. Inaddition to the removal of particles, the reduction of nitrogen oxidesalso plays an important part in exhaust-gas purification. GermanPublished, Non-Prosecuted Patent Application DE 42 03 807 A1 hasdisclosed a device for that purpose, in which an oxidizing agent isinjected and mixed with the exhaust gas.

SUMMARY OF THE INVENTION

It is according an object of the invention to provide a method forremoving soot particles from an exhaust gas, an associated collectingelement and a system for particles in an exhaust-gas flow, whichovercome the hereinafore-mentioned disadvantages of the heretofore-knownmethods and devices of this general type.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for removing soot particles froman exhaust gas of an internal combustion engine, in particular a dieselengine, which comprises providing a collecting element for freelyconducting the exhaust gas. The collecting element has a multiplicity ofdeflections and/or swirl-inducing and calming zones. The exhaust gas isfed through the collecting element. At least some of the particles areretained or swirled around long enough to ensure an adequate probabilityof a reaction with nitrogen dioxide present in the exhaust gas untillargely eliminating the collected particles.

With the objects of the invention in view, there is also provided acollecting element for collecting soot particles from an exhaust gasflowing through the collecting element. The collecting element comprisesflow paths for freely conducting the exhaust gas. The flow paths havedeflections or swirl-inducing and calming zones.

In tests with mixing elements of the type described in InternationalPublication No. WO 91/01807, corresponding to U.S. Pat. Nos. 5,045,403and 5,130,208 or International Publication No. WO 91/01178,corresponding to U.S. Pat. No. 5,403,559, for example, which were testedwith the aim of better distribution of additives injected into exhaustsystems, it has surprisingly been found that such elements can also leadto a reduction in soot particles. The particles are thought to beretained by virtue of the swirling within the mixing elements, whichoccurs at deflection points in the flow, or are as it were washedagainst the walls within the mixing element (an effect comparable togravitational separation) and then stick firmly. A possible metal/sootinteraction and/or an exhaust-gas/duct-wall temperature gradient mayalso play a part in the sticking of the particles. Pronouncedagglomeration of the particles, especially in the case of uncoated andmetallic walls, is also observed. The present invention makes use ofthese insights by configuring a collecting element in such a way thatsoot particles are retained there long enough to ensure an adequateprobability of a reaction with the nitrogen dioxide present in theexhaust gas.

The route taken is therefore not the usual one of filtering out 100% ofthe soot particles, as it were forcibly, through the use of porous wallsor the like but simply that of increasing the probability of a reactionbetween soot particles and nitrogen dioxide by extending the dwell timeof the soot particles in the collecting element. This is accomplished,while maintaining flow paths that are freely traversable per se, throughthe use of a sufficient number of swirl-inducing and calming orstabilization zones and/or through the use of deflections or deviationswhich promote deposition of the particles on the walls. While a particleflowing with the flow of exhaust gas has little chance of reacting withother constituents of the exhaust gas, its chances of doing sodramatically increase if the particle is retained in a swirl-inducingzone or is deposited on a wall. All nitrogen dioxides passing by maythen take part in a reaction and thus quickly break down the sootparticles. The collecting element can therefore not become clogged butis instead continuously regenerated.

A zone in the duct with a low flow velocity is referred to as a calmingzone, and a zone in which there is no fluid motion is referred to as adead zone.

The collecting element is referred to as being freely traversable incontrast to closed systems because there are no dead ends for the flow.As a result, the collecting element cannot become clogged like aconventional filter system, where pores can become blocked, because theflow will entrain the particles before that happens. At the same time,it is particularly advantageous that the deflections are configured insuch a way that about 2% to 15% of the particles still contained by theexhaust gas are thrown against the walls of the collecting element ineach case. A preferred range is between 4% and 8%. The phrase “particlesstill contained by the exhaust gas” takes into account the fact that thequantity of particles decreases in the direction of flow and withrelatively frequent deflection of the flow of exhaust gas. For thispurpose, the number of deflections should, in particular, be chosen insuch a way that, at least statistically, the entire flow of exhaust gasis guided against the walls of the collecting element. The quantitylimitation on the impinging particles or on the flow of exhaust gas tobe deflected has the advantage that only a very small pressure dropoccurs across the collecting element.

A conical system is preferred in order to cover various (dynamic) loadcases of the drive system of a motor vehicle. Such systems, as describedin International Publication No. WO 93/20339, corresponding to U.S. Pat.No. 5,506,028, for example, have widening ducts. That inevitably givesrise to particularly favorable conditions for the collection ofparticles at any mass flow rate at any point of the ducts if they areprovided with appropriate deflecting or swirl-inducing structures.

The preferred material for the collecting element is metal, but it canalso be an inorganic (ceramic, fiber material), organic ororganometallic plastic and/or a sintered material. The walls of thecollecting element can be coated with a wash coat and/or catalyticallyactive material or can be uncoated.

The wall thickness is preferably in a range of between 0.02 and 0.11 mm,particularly preferably between 0.04 and 0.08 mm.

The cell densities (number of ducts per unit of cross-sectional area) ofa collecting element are preferably in a range of from 25 to 1000 cellsper square inch (cpsi), preferably between 200 and 400 cpsi.

A typical collecting element has 200 cpsi, for example, a volume ofabout 0.5 to 0.8 liters per 100 kW and a geometrical surface area of 1to 2 m² per 100 kW in relation to a diesel engine. Deflecting orswirl-inducing structures are at intervals of 3 to 20 mm in the flowpaths.

The collecting element can be regenerated continuously or periodically.It is possible, in the case of a diesel-engine exhaust line, forregeneration to be achieved by oxidation of the soot either through theuse of nitrogen dioxide (NO₂) at a temperature above about 250° C. orthermally with air or oxygen (O₂) at a temperature>500° C. and/or byinjection of an additive (e.g. cerium).

Soot oxidation through the use of NO₂, using the mechanism of thecontinuous regeneration trap (CRT), for example, as follows:

C+2NO₂−>CO₂+2NO

requires that an oxidation catalytic converter be placed upstream of thecollecting element in the exhaust line to oxidize an adequate amount ofthe NO to NO₂. Studies have shown that up to twice the amount,preferably 1.2 times the amount (corresponding to about 2.4 mol), of NO₂should be fed into the collecting element as being required for completestoichiometric soot oxidation. However, the quantity ratio of thereaction partners also depends significantly on the mixing of the fluidsand therefore different quantity ratios should also be used, dependingon the configuration of the collecting element.

In the case of various embodiments, the collecting element can have manyother desired side effects when installed in a motor-vehicle exhaustline, for example:

According to one embodiment, the collecting element has the function notonly of removing particles but also of mixing the exhaust gas withanother fluid, mixing diesel exhaust gas with ammonia or urea solutionfor reduction, for example, as when using the SCR (selective catalyticreduction) method, for example. For this purpose, the collecting elementis combined with at least one additive feed.

The distribution of the flow of the fluid flowing through is alsooptimized, for subsequent feeding into a reduction catalytic converter,for example, through the use of an advantageous refinement of thecollecting element. According to one embodiment, the collecting elementis used in combination with an upstream additive feed.

According to one embodiment, the collecting element is used incombination with at least one catalytic converter. Suitable catalyticconverters and/or precatalytic converters to be used for this purposeare, in particular: oxidation catalytic converters, heated catalyticconverters with an upstream or downstream heating disk, hydrolysiscatalytic converters and/or reduction catalytic converters. Theoxidation catalytic converters being used include those which oxidizethe NO_(x) (nitrous gases) to nitrogen dioxide (NO₂) as well as thosewhich oxidize the hydrocarbons and carbon monoxide to CO₂. The catalyticconverters are tubular or conical.

It is also advantageous to combine the collecting element with at leastone catalytic converter and at least one turbocharger or to combine acollecting element with a turbocharger. The collecting elementdownstream of the turbocharger can be disposed close to the engine or inan underfloor position.

The collecting element is also used in combination with an upstream ordownstream soot filter. It is possible for the downstream soot filter tobe substantially smaller than a conventional soot filter because it onlyhas to offer an additional safeguard, ensuring that particulateemissions are eliminated. A filter with an area of 0.5 to 1 m² per 100kW of a diesel engine is sufficient, for example, whereas filter sizesof about 4 m² per 100 kW are required without a collecting element.

The following examples give configurations that demonstrate the largenumber of possible combinations of the collecting element with catalyticconverters, turbochargers, soot filters and an additive feed along anexhaust line of a motor vehicle:

A) Oxidation catalytic converter-turbocharger-collecting element. It ispossible for the collecting element to be disposed close to the engineor in an underfloor position.

B) Precatalytic converter-collecting element-turbocharger.

C) Oxidation catalytic converter-turbocharger-oxidation catalyticconverter/collecting element.

D) Heated catalytic converter-first collecting element-second collectingelement (in which the first collecting element and the second collectingelement may be the same or different).

E) First collecting element-conical opening of the exhaust line-secondcollecting element.

F) Additive feed-collecting element-hydrolysis catalyticconverter-reduction catalytic converter.

G) Precatalytic converter-oxidation catalytic converter-additive feed(possibly soot filter)-collecting element, e.g. in conical form, ifrequired with hydrolysis coating-(possibly soot filter)-(possiblyconical to increase the cross section of the tube) reduction catalyticconverter.

The collecting element can have various coatings, each of whichdetermine one function, depending on the embodiment. In addition to thestorage, mixing and flow-distribution function, the collecting elementcan also act as a hydrolysis catalytic converter, for example.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for removing soot particles from an exhaust gas, anassociated collecting element and a system, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a block diagram of an exhaust system of an internal combustionengine;

FIG. 2 is a fragmentary, diagrammatic, perspective view of an embodimentof a collecting element according to the invention; and

FIG. 3 is a fragmentary, perspective view of another exemplaryembodiment of a collecting element according to the invention.

DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen an internal combustionengine 1, in particular a diesel engine, the exhaust gases of which arefed into an exhaust line 2. The exhaust line has at least one oxidationcatalytic converter 3, which can also be preceded by a non-illustratedprecatalytic converter disposed very close to the outlet of the internalcombustion engine. A turbocharger 7 can also be disposed in that region.Alternatively, a turbocharger 8 or 9 can be used. An infeed 4 for anadditive, in particular urea, is furthermore disposed further downstreamin the exhaust line 2. A collecting element 5 according to the inventionis disposed downstream of the infeed 4 and is followed, in turn, by anSCR catalytic converter 6. The collecting element 5 is illustrated asbeing conical, but could be tubular.

FIG. 2 shows a small portion of an exemplary embodiment of a collectingelement 5 according to the invention. The collecting element isconstructed from structured sheet-metal layers 11 and smooth sheet-metallayers 12. The layers 11, 12 define freely traversable flow paths 13which are, however, provided with deflections 15, 16 due to a specialconstruction of the structured sheet-metal layers 11. These deflectionsimpart a swirl to the exhaust gas, as a result of which particles stayin the collecting element 5 longer and can react more easily with othercomponents of the exhaust gas. Depending on the precise configuration ofthe deflections 15, 16, they may also throw particles against the wallsof the flow paths 13 formed by the sheet-metal layers 11, 12, where theystick. The deflections 15, 16 have aperture angles or angles ofincidence of 20° to 90°, for example. The larger the angle of incidenceto the direction of flow, the greater the deflection and swirling whichis achieved, although it is associated with an exponential rise in thepressure loss. The optimum angle is between 40° and 50°, which providesa good swirl for an acceptable pressure loss. The deflections 15, 16 arepreferably combined with openings 17 in the structured sheet-metallayers 11, giving rise to more powerful vortices and ensuring that theflows in adjacent flow paths 13 mix.

Another exemplary embodiment is illustrated in FIG. 3, again only as asmall portion of a collecting element 5. The collecting element isconstructed from a structured sheet-metal layer 21 and a smoothsheet-metal layer 22 with apertures 28. The layers 21, 22 define freelytraversable flow paths 23. Wing-like deflections 25, 26 in combinationwith curved openings 27 lead to the same effects as those describedabove. The apertures 28 in the smooth sheet-metal layers 22 assist inthe formation of vortices and the mixing of exhaust gases in thecollecting element.

It has been observed that the deposition of particles takes placeespecially in the region of the inlet and outlet surfaces of thecollecting elements. Therefore, in accordance with one embodiment, thecollecting element is employed in the form of a plurality of narrowhoneycomb bodies as disk elements connected in series. This producesdeflections and/or swirl in the inlet and outlet region of each disk. Upto 10 elements is the preferred number.

The present invention proposes a collecting system for soot particleswhich can replace conventional filter systems and provides majoradvantages as compared with those systems:

On one hand, it cannot become clogged, and the pressure drop caused bythe system does not increase as rapidly over its operating life as withfilter systems because the particles stick outside the flow of fluid. Onthe other hand, it causes comparatively small pressure losses because itis an open system.

1. A collecting element for collecting soot particles from an exhaustgas flowing through the collecting element, the collecting elementcomprising: flow paths for freely conducting the exhaust gas, said flowpaths having at least one of deflections and swirl-inducing and calmingzones.
 2. The collecting element according to claim 1, furthercomprising at least one metallic honeycomb body having sheet-metallayers, said sheet-metal layers being at least partly structured to formsaid flow paths.
 3. The collecting element according to claim 2, furthercomprising obstacles in said flow paths.
 4. The collecting elementaccording to claim 3, wherein said sheet-metal layers define walls ofsaid flow paths, and at least one of said obstacles and said deflectionseach throw between 2% and 15% of the particles still contained by theexhaust gas against said walls.
 5. The collecting element according toclaim 4, further comprising a coating on at least some of said walls ofsaid flow paths.
 6. The collecting element according to claim 4, furthercomprising a wash coat of aluminum oxide on at least some of said wallsof said flow paths.
 7. The collecting element according to claim 1,further comprising a conical housing containing said flow paths.
 8. Anengine and exhaust system, comprising: an internal combustion engine; anexhaust line connected to said internal combustion engine; and at leastone collecting element in said exhaust line for collecting sootparticles from exhaust gas flowing through said at least one collectingelement, said at least one collecting element including flow paths forfreely conducting the exhaust gas, said flow paths having at least oneof deflections and swirl-inducing and calming zones.
 9. The systemaccording to claim 8, wherein said internal combustion engine is adiesel engine.
 10. An exhaust system, comprising: a collecting elementfor collecting soot particles from an exhaust gas flowing through saidcollecting element, said collecting element including flow paths forfreely conducting the exhaust gas, said flow paths having at least oneof deflections and swirl-inducing and calming zones; and at least oneadditive feed communicating with said collecting element.
 11. The systemaccording to claim 10, wherein said at least one additive feed isdisposed upstream of said collecting element.
 12. The system accordingto claim 10, wherein said at least one additive feed is disposeddownstream of said collecting element.
 13. An exhaust system,comprising: a collecting element for collecting soot particles from anexhaust gas flowing through said collecting element, said collectingelement including flow paths for freely conducting the exhaust gas, saidflow paths having at least one of deflections and swirl-inducing andcalming zones; and at least one catalytic converter communicating withsaid collecting element.
 14. The system according to claim 13, whereinsaid at least one catalytic converter includes at least one oxidationcatalytic converter oxidizing nitrous gases to yield nitrogen dioxide.15. The system according to claim 14, wherein said at least oneoxidation catalytic converter is disposed upstream of said collectingelement.
 16. The system according to claim 14, wherein said at least oneoxidation catalytic converter is disposed downstream of said collectingelement.
 17. An exhaust system for an internal combustion engine,comprising: a collecting element for collecting soot particles from anexhaust gas flowing through said collecting element, said collectingelement including flow paths for freely conducting the exhaust gas, saidflow paths having at least one of deflections and swirl-inducing andcalming zones; and at least one turbocharger communicating with saidcollecting element.
 18. The system according to claim 17, wherein saidcollecting element is mounted close to the engine.
 19. The systemaccording to claim 17, wherein said collecting element is mounted in anunderfloor position.
 20. The system according to claim 17, wherein saidcollecting element is mounted in an underfloor position close to theengine.
 21. The system according to claim 17, wherein said at least oneturbocharger is disposed downstream of said collecting element.
 22. Thesystem according to claim 17, wherein said at least one turbocharger isdisposed upstream of said collecting element.
 23. The system accordingto claim 17, wherein the engine is a diesel engine, an exhaust line isconnected to the diesel engine, said at least one turbocharger isconnected in said exhaust line upstream of said collecting element, andan oxidation catalytic converter is connected in said exhaust linedownstream of said at least one turbocharger.
 24. The collecting elementaccording to claim 1, wherein said flow paths have at least one ofopenings and apertures for ensuring the mixing of flows in adjacent onesof said flow paths.
 25. The collecting element according to claim 1,wherein said flow paths are configured for at least one of retaining atleast some of the particles and swirling around at least some of theparticles long enough to ensure an adequate probability of a reactionwith nitrogen dioxide present in the exhaust gas until substantiallyeliminating the particles.
 26. The system according to claim 10, whereinsaid collecting element is filterless.